In devices with luminous indications, displays, or means for projection, the readability of the indication or the representation quality of images can vary considerably with the brightness of the ambient light. Under changing light conditions, the user generally has the opportunity to adapt specific operating parameters of the device, such as brightness or contrast, for instance, to the light conditions in the vicinity of the apparatus. In order to spare the user manual adaptation, various apparatus already have automatic adaptation of specific operating parameters to the brightness of the ambient light. The detection of the ambient brightness that is required for automatic adaptation is usually based on an optical sensor fitted in the housing of the apparatus, generally pointing in the direction of the observer. The signal made available by the ambient light sensor is fed to a regulating arrangement which automatically performs optimum adaptation of the operating parameters to the ambient brightness. The application of such automatic adaptation is appropriate, inter alia, in televisions, video projection devices, dashboard illuminations or devices with liquid crystal displays such as portable computers or mobile telephones.
Although the above-described method for adapting the operating parameters of an indication or projection apparatus to the ambient brightness is already used in commercially available apparatus, at the present time it is employed only in a small number of, usually high-quality, devices. The reason for this is that the sensor required for measuring the ambient brightness causes relatively high additional product costs, on the one hand because of the costs for the component per se but on the other hand because of the precautions which are necessary for incorporating the sensor in the apparatus housing. In addition, the optical sensor can entail restrictions for the configuration of the apparatus housing which, under certain circumstances, cannot be harmonized with a miniaturization or a specific aesthetic configuration of the housing.
It is desirable because of this to provide a circuit for detecting the ambient brightness with the least possible additional technical outlay.